The present invention relates to a servo control system for a drive motor of an impact printer, facsimile transceiver or the like.
U.S. Pat. No. 3,839,665 to Andrew Gabor discloses an "APPARATUS MEASURING RELATIVE VELOCITY OF MOVABLE MEMBERS INCLUDING MEANS TO DETECT VELOCITY FROM THE POSITION ENCODER" in which a transducer coupled to a motor shaft provides both position and velocity information for use in a closed loop control system. The transducer includes a disc with deposited metallic parallel conductors each conductor carrying current in a direction opposite to the adjacent conductor. Relative movement of one disc with respect to another produces a position signal in a manner well known in the art. However, multiple windings are used to provide multiple position signals displaced in space phase which are then processed by differentiation and commutation to provide a velocity signal used as a control voltage in the control loop. A reference signal is also derived by commutation from the position signal.
Whereas the overall principle of Gabor's device provides an effective servo control system, the transducer comprising the various conductors and windings is complicated and expensive to produce.
The present invention utilizes a greatly simplified but efficient transducer comprising optoelectronic elements. A light emitting diode illuminates first and second photodiodes through an occluder disc which is formed with a plurality of circumferentially spaced, light transmitting apertures. The disc is driven by a servo motor shaft so as to alternately cover and uncover the photodiodes and produce electrical position signals. Due to relative spacing, the signals produced by the first and second photodiodes are 90.degree. out of phase relative to each other and are electrically essentially similar to those produced by Gabor's transducer.
Although the present transducer operates effectively even at extremely high speeds, a problem is encountered due to drift of the electrical characteristics of the photodiodes caused by variations in ambient temperature, natural deterioration over a prolonged period of use and the like. For example, if constant current is supplied to the light emitting diode and the ambient temperature is varied by 50.degree. C., the output voltages of the photodiodes may vary by as much as 40%. Since the position signals are differentiated to produce the velocity signal, the amplitude of the velocity signal corresponding to the motor shaft velocity, any variation in the output voltages of the photodiodes will produce a significant error in the operating speed and positioning accuracy of the servo system.